Photoneutralization of positively charged atoms and molecules, as a means of forming nigh kinetic energy neutral beams, was investigated ten years ago for use in magnetic fusion. This approach has developed slowly, if at all, in the sunsequent years, due to lack of sufficient development of the required laser and optical systems. The subject invention offers improvements in the optical system efficiency of photoneutralization apparatus.
Of all the methods for producing high energy neutral beams, photoneutralization of energetic negative ions is the most attractive. Neutralization efficiencies of 80-85 percent are routinely achieved with little or no degradation of the kinetic energy of the beam particles and with production of only a small percentage of impurities. Additionally, photoneutralization costs are $1-$2 per watt of neutral beam output.
Photoneutralization was first proposed by the inventor hereof in 1975 as a means of forming neutral beams for magnetic fusion (J. H. Fink and A. M. Frank, UCRL-16844, LLL Report (1975) ). At that time, the inventor hereof proposed use of solid state gallium arsenide lasers to strip the excess electrons from the negative ions in a multi-pass arrangement within an optical cavity. A gallium arsenide laser is attractive because the light emitted (at .lambda.=0.85 .mu.m) is close to the wavelength for maximum photoneutralization cross section of a number of negative ions such as H.sup.- and C.sup.-, as indicated in FIG. 4. A gallium arsenide laser operates at efficiencies of 20-40 percent, which is also quite attractive. However, gallium arsenide lasers may still require considerable development before they can be used reliably in any application. Among other things, such a laser must operate efficiently at room temperature, with an angular divergence that is greatly reduced from the present divergence of such lasers.
Another attractive laser, which has received far more development in the last eight years, is atomic iodine, which emits radiation at a wavelength .lambda.=1.31 .mu.m. This wavelength is not substantially coincident with the wavelength of maximum photoneutralization cross section for any of the negative ions mentioned above; but the associated photoneutralization cross section at the iodine emission wavelength is about half of its maximum value, and the use of such an infrared wavelength is less likely to produce undesirable impurities in the laser gas. Further, an atomic iodine laser has an associated efficiency of operation of 7-11 percent, which is respectable.
The maximum photoneutralization cross section for negative ions is of the order of .sigma..sub.ph =10.sup.-17 cm.sup.2. With a beam charged particle density of, say, n=10.sup.15 cm.sup.-3 present, the characteristic absorption length for the photoneutralization process is d=(n.sigma.).sup.-1 =100 cm. Therefore, if one is to ensure virtually complete use of the photoneutralization laser radiation, this radiation must travel 460 cm or more within the portion of the space through which the charged particles pass. This appears to require multiple passage of the radiation through the portion of the space through which the charged particles pass, to assure reasonably complete (99 percent) absorption of the photoneutralization radiation.